1. Field of the Invention
This invention relates to an optical disk drive apparatus used for data or information memory systems for recording onto and playing back from the surface of a disk recording medium data signals, by using a light beam, for example, a laser beam. More particularly, the invention relates to an optical pick-up head actuator which carries an optical system, and guides a light beam to a target recording track of an optical disk.
2. Description of the Prior Art
The conventional optical disk drive apparatus is provided with a focusing actuator for moving an objective lens in a direction perpendicular to a recording surface of an optical disk to focus a light beam (e.g. laser beam) upon the recording surface; a tracking actuator for moving the objective lens in a direction parallel to the disk and transverse to the recording tracks to let the light beam follow a target track; and a seeking actuator (ordinary linear motor) for carrying an optical pick-up head block, which includes the focusing actuator, the tracking actuator and other optical components and devices (such as a mirror, a prism, a semi-conductor laser) and a detector, in a radial direction of the disk for seeking the target track to be read or written. The weight of the optical pick-up head block is usually heavy because of some heavy components mounted thereon, especially magnetic circuits of the focusing and tracking actuators. The total moving mass including the mass of means for carrying the optical pick-up head block in the seeking actuator is approximately between 200 and 1000 grams. Therefore, it is very difficult for the conventional apparatus to achieve a high speed track access. If the optical pick-up head block was subjected to an acceleration of 10G (G is the acceleration of gravity), a powerful thrust about 20-100N (Newton) would be required for the seeking actuator. A linear voice coil motor, for example, meets this requirement, but it is very large in size and weight. With these actuators, the optical disk drive apparatus would hardly be made compact or light, and would hardly diminish power dissipation.
On the other hand, the large acceleration for achieving high speed access to the target track brings about various difficulties. The objective lens is usually suspended by flexible members in the magnetic circuit of the focusing actuator to be moved smoothly in a direction perpendicular to the disk surface. The objective lens is also usually suspended by other flexible members directly or indirectly (through the focusing actuator) in the magnetic circuit of the tracking actuator and is also moved smoothly in the direction of the track crossing. Therefore, the large acceleration causes the objective lens to vibrate furiously. This makes the optical axis of the objective lens incline transiently, and continue for a while after access. Consequently, with the large acceleration the optical system becomes unstable, for example, unable to focus, and takes a long time for setting after access.
Another prior art approach to the optical pick-up head block is shown in Japanese Laid-open Application 50-34507 by Koizumi for an optical disk drive. Koizumi proposes an optical pick-up head block that has compact and low-weight moving portions, which can easily replace a laser beam source. Embodying the objective above, the laser beam source is mounted on the stationary portions, and the optical pick-up head block comprises a vacant prop and a vacant arm which is attached to the prop. The vacant arm is able to rotate around the axis of the prop, and is equipped with optical devices such as lenses, prisms, and mirrors. The laser beam passes through the vacant prop. Despite these proposals, there will exists many difficulties for obtaining a high speed track access. One is that the moving portions' mass is still large, because the vacant arm includes many optical components and a focusing means (for example, a focusing actuator) which are not mentioned. Another is that high stiffness is hardly achieved with the vacant arm, because the vancant low weight arm has to be comprised of thin and light materials. Therefore, the powerfull acceleration causes the moving portions including the arms to undergo serious vibration. Moreover, because of a lack of stiffness, the closed-loop servo control cannot achieve very accurate trackability for track following.